In recent years, a flexible device in which a functional element such as a semiconductor element, a display element, or a light-emitting element is provided over a substrate having flexibility (hereinafter also referred to as a flexible substrate) has been developed. Typical examples of the flexible device include, as well as a lighting device and an image display device, a variety of semiconductor circuits including a semiconductor element such as a transistor.
As a method for manufacturing a device using a flexible substrate, a technique has been developed in which a functional element such as a thin film transistor or an organic EL element is formed over a formation substrate (e.g., a glass substrate or a quartz substrate), and then the functional element is transferred to a flexible substrate. This technique needs a step of separating a layer including the functional element from the formation substrate (also referred to as a separation step).
A technique has been proposed in which an element formed over a glass substrate is separated from the substrate and transferred to another base material such as a plastic film.
Separation and transfer techniques described in Patent Documents 1 and 2 are proposed. Patent Document 1 discloses a separation technique in which a silicon oxide film serving as a separation layer is removed by wet etching. In addition, Patent Document 2 discloses a separation technique in which a silicon film serving as a separation layer is removed by dry etching.
A separation and transfer technique described in Patent Document 3 is also proposed. Patent Document 1 discloses a technique in which a metal layer (Ti, Al, Ta, W, Mo, Cu, Cr, Nd, Fe, Ni, Co, Ru, Rh, Pd, Os, or Ir) is formed over a substrate and an oxide layer is stacked thereover; at this time, a metal oxide layer of the metal layer is formed at an interface between the metal layer and the oxide layer, and separation is carried out in a later step by utilizing this metal oxide layer.